netdev-dpdk: Add per virtqueue statistics.

The DPDK vhost-user library maintains more granular per queue stats
which can replace what OVS was providing for vhost-user ports.

The benefits for OVS:
- OVS can skip parsing packet sizes on the rx side,
- dev->stats_lock won't be taken in rx/tx code unless some packet is
  dropped,
- vhost-user is aware of which packets are transmitted to the guest,
  so per *transmitted* packet size stats can be reported,
- more internal stats from vhost-user may be exposed, without OVS
  needing to understand them,

Note: the vhost-user library does not provide global stats for a port.
The proposed implementation is to have the global stats (exposed via
netdev_get_stats()) computed by querying and aggregating all per queue
stats.
Since per queue stats are exposed via another netdev ops
(netdev_get_custom_stats()), this may lead to some race and small
discrepancies.
This issue might already affect other netdev classes.

Example:
$ ovs-vsctl get interface vhost4 statistics |
  sed -e 's#[{}]##g' -e 's#, #\n#g' |
  grep -v =0$
rx_1_to_64_packets=12
rx_256_to_511_packets=15
rx_65_to_127_packets=21
rx_broadcast_packets=15
rx_bytes=7497
rx_multicast_packets=33
rx_packets=48
rx_q0_good_bytes=242
rx_q0_good_packets=3
rx_q0_guest_notifications=3
rx_q0_multicast_packets=3
rx_q0_size_65_127_packets=2
rx_q0_undersize_packets=1
rx_q1_broadcast_packets=15
rx_q1_good_bytes=7255
rx_q1_good_packets=45
rx_q1_guest_notifications=45
rx_q1_multicast_packets=30
rx_q1_size_256_511_packets=15
rx_q1_size_65_127_packets=19
rx_q1_undersize_packets=11
tx_1_to_64_packets=36
tx_256_to_511_packets=45
tx_65_to_127_packets=63
tx_broadcast_packets=45
tx_bytes=22491
tx_multicast_packets=99
tx_packets=144
tx_q0_broadcast_packets=30
tx_q0_good_bytes=14994
tx_q0_good_packets=96
tx_q0_guest_notifications=96
tx_q0_multicast_packets=66
tx_q0_size_256_511_packets=30
tx_q0_size_65_127_packets=42
tx_q0_undersize_packets=24
tx_q1_broadcast_packets=15
tx_q1_good_bytes=7497
tx_q1_good_packets=48
tx_q1_guest_notifications=48
tx_q1_multicast_packets=33
tx_q1_size_256_511_packets=15
tx_q1_size_65_127_packets=21
tx_q1_undersize_packets=12

Reviewed-by: Maxime Coquelin <[email protected]>
Signed-off-by: David Marchand <[email protected]>
Signed-off-by: Ilya Maximets <[email protected]>